The present invention relates to a cold cathode mercury vapor discharge lamp having a cold cathode and used in liquid crystal apparatus or for backlighting an automobile instrument or the like.
Lamps that use cold cathodes as the electrodes generate a small amount of heat at their electrodes and therefore have a long lamp life, and can also have tube diameters of 15 mm or less since there is a small amount of bulb heating. They are therefore often used for compact fluorescent lamps.
In recent years, the increasing compactness of devices has meant that it has become necessary for this type of cold cathode mercury vapor discharge lamp to have an improved brightness without increasing the size of the cold cathode discharge lamp. One method of improving the brightness is by increasing the discharge current.
However, the maximum permissible current that is supplied to the electrode is determined by the surface area of the electrode and if a discharge current that exceeds this permissible current is supplied, then ion impacting becomes larger, causing an increase in the amount of heat generated at the electrode. Also, because cold cathode mercury vapor discharge lamps generally use the glow discharge region, an increased current causes spattering of the electrode material and consequent deterioration that shortens the life of the electrodes. One means of countering this is by increasing the electrode surface area.
FIG. 1 shows a prior art device made by a method whereby the electrode surface area is increased. This method involves applying a fluorescent substance to the inside walls of the glass bulb 1, baking the bulb and sealing the stems 2 at each end. In addition, to these stems 2 are applied an exhaust tube 3 and through each of the stems 2 are sealed two stem leads 4, each of which is formed of an inner wire 5 and an outer wire 6. In addition, the distal ends of the inner wires 5 have welded to them a V-shaped plate electrode 7. The surfaces of this metal plate 7 have mounted thereon a mercury alloy and a getter.
However, with the cold cathode mercury vapor discharge lamp shown in FIG. 1, the effective electrode surface area is not large and impure substances are formed resulting in the problem of blackening where the electrode material is spattered and adheres to the inner surface of the fluorescent tube. In order to ameliorate this problem, as shown in FIG. 2, for example, two metal plates having different bent angles are welded to the distal ends of the inner wires 5 so that a shape that is fan shape in section is formed. Such an arrangement is shown in Japanese Laid Open Utility Model Application No. 2-56344/1990.
With the configuration shown in FIG. 2, there is little of the above described blackening, but it is impossible to completely prevent such blackening from occurring. In addition, because the gaps between the electrodes are narrow, it is not possible for the electrons to reach the gaps or spaces therebetween. Therefore, the effective area of the electrodes is small.
Also, in such mercury vapor discharge lamps having a compact cold cathode, there is provided a slit portion that allows light to pass along the axis of the bulb, with only light from the slit portion being emitted in a specific direction. These lamps are called aperture type lamps.
In such lamps, the cold cathode mounted at the end of the bulb is formed by welding a cold electrode comprising a metal plate such as nickel, or the like, to wires made of Jumet such as JEMDES (the trade marks of SAES Co. Ltd.) or the like, and with the cold electrode plate formed to a V-shape or a cylinder shape. To surfaces of the cold cathode are mounted a mercury discharge unit that discharges mercury into the discharge space after the lamp has been completed, and a getter comprising phosphorous and barium and a metal such as mercury absorbing indium or the like.
However, when the mercury discharge unit is added to the cold cathode, it is positioned opposite to the aperture portion of the bulb, and when the mercury discharge unit is heated by high-frequency induction to discharge the mercury inside the bulb, the discharged mercury attaches to the aperture (slit) portion.
This adhered mercury causes blackening of the aperture portion and causes such problems as obstructing the passage of light and adversely influencing the distribution of the output light.